1. Field of the Invention
The present invention relates in general to the field of telecommunications, and more particularly, to an enhanced High Voltage Line Interface (HVLI) circuit.
2. Description of the Prior Art
Subscriber line interface circuits (SLIC) are customarily found in the central office exchange of a telecommunications network. The SLIC weds the digital switching network of the central office exchange to a plurality of analog subscriber lines. The analog subscriber lines connect to subscriber stations or telephone instruments found at subscriber locations remote from the central office exchange.
The SLIC functions to supply power to a subscriber station and to transmit and receive voice signals between the digital switching network and the subscriber station.
Modern solid state SLICs are constructed using specialized integrated circuits. This construction dispenses with the need for inductive components in the analog end of the interface. The operating environment of the SLIC includes a necessity to provide high voltages and currents, used for analog voice transmission and substation signalling, as well as, low voltage digital logic signals used for the transmission of digital data between the SLIC and the digital switching network.
In many presently known telephone line circuits the battery feed function has been performed by using a passive, highly balanced, split winding transformer and or inductors which carry up to 120 ma dc. This passive circuit has a wide dynamic range, passing noise-free differential signals while not overloading with the 60 Hz longitudinal induced currents. The line circuit just described, feeds dc current to the subscriber loop and also provides the voice path for coupling the voice signal between the subscriber station and the central office. The electromagnetic components of passive line circuits are normally bulky and heavy and consume large amounts of power for short subscriber loop lengths where the current fed to the subscriber station is more than necessary for equalization. Active line-feed circuits can be less bulky and require lower total power, but meeting dynamic range and precision balance requirements dictates an overly complex circuit design.
Recently, solid state replacements for the electromagnetic components of the aforementioned line circuits have been developed. Such as the High Voltage Line Interface (HVLI) circuit described in patent application Ser. No. 445,516, filed Dec. 4, 1989, entitled "High Voltage Subscriber Line Interface Circuit", having a common assignee with the present invention.
Several other devices, such as, high voltage bipolar transistors and other specialized integrated circuits are being designed to replace the heavy and bulky components of the electromagnetic line circuit. Such a device is described in the IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-16, NO. 4, August 1981, entitled, "A High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface." These smaller and lighter components allow the manufacture of telephone switching systems having more line circuits per circuit card as well as decreasing the physical size of the switching system.
However, presently known high voltage ICs, still suffer from deficiencies in meeting good transmission performance specifications. These deficiencies manifest themselves in poor longitudinal balance and poor longitudinal current susceptibility, which cause the circuit to fail or to become noisy. Other problems presently encountered are excessive power dissipation at short loops that consume prodigious amounts of central office power and 2 wire input impedance circuits that are complex and/or that exhibit poor return loss.
Accordingly, it is an object of the present invention to provide an enhanced high voltage line interface circuit that will effectively and efficiently couple a subscriber station apparatus to a telephone switching system.